In April 2009, the Centers for Disease Control and Prevention (CDC) reported the first two cases of human infection with a novel influenza A H1N1 virus in the United States and over the course of the next several weeks, the virus spread around the world prompting the World Health Organization (WHO) to declare a global influenza pandemic on June 11, 2009. Antigenic and genetic characterization of the novel influenza A H1N1 virus indicated that the virus contained a unique combination of gene segments from North American and Eurasian lineages of swine influenza viruses. The viruses isolated from human cases of the 2009 pandemic H1N1 virus infection were antigenically homogeneous and antigenically similar to North American swine H1N1 viruses, but distinct from seasonal human influenza A H1N1 viruses. Based on antigenic and genetic analysis and epidemiologic information, the WHO selected the A/California/7/2009 (H1N1) virus as the reference virus for vaccine development;inactivated and live attenuated vaccines for pandemic H1N1 were licensed in 2009. We initiated clinical studies to evaluate the immunogenicity and infectivity of the licensed inactivated virus and live attenuated H1N1 vaccines in different age groups. Analysis of the data from these studies is in progress.